This invention relates generally to transport systems for sheetform media of magnetic and other types, and, more specifically, to an arrangement for enhancing contact at high speeds between a signal exchange head and a magnetic medium.
Increasing demands on data storage rates and capacities require modern tape drives to record in full contact between the head and the tape at relatively high speeds. In general, recording in contact minimizes losses due to the spacing between head and tape and enables higher recording densities (Hinteregger and Muftu). (This paper is incorporated fully by reference herein.) (A full citation to all references is at the end of this specification.) Contact is typically generated by wrapping a moving tape under tension over a curved geometry for the signal exchange surface. In such a situation, the contact pressure that arises is generally proportional to the ratio of the applied tension T to the radius of curvature R of the surface. Thus, increasing T and reducing R increases the contact pressure. Heads that are used for tape are typically made of a hard, ceramic material, and are designed to withstand the abrasive conditions of high speed tape use under such contact pressures.
Recording heads manufactured with the thin film (TF) semi-conductor technology are extensively used with magnetic hard disk media. These heads have signal processing properties that make them attractive for high speed, high signal density uses. However, at high contact pressures, thin film heads, just like ceramic heads made in conventional ways, experience head wear at the signal exchange site and recession at the magnetic poles, which can become a problem. Thin-film heads typically tolerate less than 1 .mu.m wear and 30 nm of recession. Wear resistant head materials and tape bearing surface overcoats can improve the head's wear life. Wear rate tends to be inversely proportional to the hardness of the material (Bhushan and Lowry; Kawakubo and Yahisa) and proportional to contact pressure. But experiments show that wear is not proportional to contact pressure and is sublinear at sufficiently low contact pressures (Bhushan Handbook). Therefore, the design of a contact-recording-head should include a hard load bearing surface, and guarantee contact pressures consistent with preventing separation of the tape from the head, and low enough to provide an acceptable head life (if not to arrest wear completely).
Some geometries, such as are shown schematically in FIGS. 1, 1A and 1B, promote what is known as "self-lubrication." As shown in FIG. 1B, the curvature of the signal exchange surface 12 is smoothly continuous, with no slope discontinuities. In other words, d.sup.2 .delta./ds.sup.2 is continuous, where .delta. is the mathematical function describing the head and s is the coordinate axis along the head. The tape is typically wrapped around the head so that it follows the tangent to the surface at the point Z of separation. Such a wrapping is referred to herein as a "tangential wrap".
The tape and the unwrapped part of the surface form a gently converging channel, such that a relatively large volume of air is entrained therebetween. Air that is entrained at E between the tape 10 moving in the direction S and the signal exchange surface 12 of the head 14 forms a cushion that separates the two from each other over large extents of the head. When this occurs, it is said that the tape "flies". This is undesirable, as increases in the distance between the tape and the magnetic gap at the signal exchange site 16 cause exponential decreases in the strength of the read back signal. According to the Wallace equation, approximately 55 dB per wavelength .lambda. of the recorded signal is lost as a result of this distance increase. The separation shown in FIG. 1B is greatly exaggerated. An actual spacing of only 0.18 .mu.m is enough to reduce the amplitude of a recording with a wave length of 1 .mu.m by a factor of ten.
Although separation of the tape from the head due to the self-lubricating effects of entrained air ("flying") is, in general, undesirable, it is also undesirable to apply a large tension to the moving tape to dramatically overcome this air lubrication effect, because that may result in higher undesirable head wear rate, discussed above. Wear would be a particularly significant problem with relatively soft thin film layers. What is desired is to achieve a balance of the two effects: making sure that the tape would always be in contact with the head, with a minimal amount of contact pressure.
One known way to help to balance these effects is to introduce one or more grooves or relieved areas 18 into the head material, near to the upstream corner. As shown FIG. 1A, the entrained air is gradually compressed as it is drawn along between the tape 10 and the signal exchange surface 12 of the head 14, and expands into the cavity 18 shown at the arrow x. This expansion results in a reduction in pressure, sometimes even to a level below ambient, so that the net total pressure on the tape 10 downstream of the cavity 18, due to the surrounding air, is toward the head surface 12. The effect of this reduced pressure lessens as the distance downstream from the cavity 18 increases, and in some cases, additional grooves are used.
The use of such a groove provides the opportunity for more control over the balancing of the air and tension related contributions to the contact pressure between the tape and the head at different locations along the dimension of tape travel. However, radius lapping in particular, and to a lesser extent machining grooves, in the very hard materials used for modern heads, is difficult and costly.
In thin film heads, the signal exchange elements are made of layers that must have a certain minimum thickness to function. These layers must also be a certain minimum distance away from the main surface of the head, where the tape or other medium contacts it. Further, with curved surface heads, the surface must be machined (sometimes lapped) to a precise shape. However, as the surface is formed by removing surface material, it is difficult to achieve the required surface, while also maintaining the required thickness of the signal exchange elements, as they might be inadvertently removed during the shaping process. Further, if excessive material is removed during the shaping process, the remaining "buffer" might wear away too quickly during use, thereby exposing the signal element to excessive wear. Therefore, if thin film technology is to be used it is desirable to use a head surface that requires only minimal shaping.
The relative speed between the media and the head also has an impact on the contact conditions. In general, for a moving tape that is tangentially wrapped around a conventional head that has a smoothly curved surface, as described above, as the speed of the tape increases, the spacing between the tape and the head increases (Gross). This is due to the increased effect of the positive air bearing as tape speed increases. It is desirable for the spacing between the head and the tape to remain at an irreducible minimum, independent of effective speed. This is the definition of "contact recording". The general trend in industry is toward faster and faster tape speeds. Thus, it is a drawback if the tape/head spacing increases with increasing speed, within the desired speed range.
It is also common for linear tape drives to operate in both a forward and a reverse direction. Therefore, it is desirable that any arrangements to address the concerns discussed above work equally well in both the forward and reverse directions.
The problems of contact recording on flexible tape media are also present in rotating media (floppy disk) and rotating head (helical and transverse scan videotape recording) applications.
In the case of floppy disks and other rotating media, the disk rotates, passing by a head, which is stationary along the circumferential direction of media rotation, but which moves radially.
It is known to provide a disk head within a surrounding, adjoining structure that includes grooves, and other relieved formations, to counteract or counterbalance the self lubrication induced "flying" effect of entrained air. However, these structures require additional components. Further, these components must be made of hard material, so that they do not wear out quickly. Consequently, they must also be fabricated from hard material that is relatively difficult and costly to machine. Further, some of them are only useful in a single direction and would not be useful in the context of a bidirectional tape drive.
Accordingly, for the foregoing reasons, there is a need for a head and tape transport arrangement that can provide a direct contact between the various types of magnetic sheet media and the head, with minimal contact forces therebetween. There is also a need to prevent separation due to entrained air, without requiring a complex (radiused and grooved) head contour and/or structure adjoining the head.
Thus, it is an object of this invention to provide a head-tape interface that would keep the sheet media in contact with the head, without increasing the tendency of the head to wear. A further object is to achieve these goals with a simple head surface that does not require complicated radiused, relieved or grooved surfaces. Another object is to enable the use of heads having a substantially flat surface. Another object is to allow increased relative media to head speeds, without increasing spacing loss or decreasing head life. It is also an object to provide these advantages for a bi-directional tape, and also for rotating media, and also for rotating head.